It is known that drilling fluids for sinking wells in rock and bringing up the rock cuttings are flowable water- or oil-based systems that are thickened to a limited extent. Oil-based systems are acquiring increasing significance in practice and are used in particular in offshore drilling operations. Oil-based drilling fluids are generally used as so-called invert emulsion muds which consist of a 3-phase system: oil, water and fine-particle solids. Drilling fluids such as these are preparations of the w/o emulsion type, i.e. the aqueous phase is heterogeneously and finely dispersed in the continuous oil phase. A range of additives may be used for stabilizing the system as a whole and for establishing the required performance properties, including in particular emulsifiers and emulsifier systems, weighting agents, fluid loss additives, viscosity adjusters and optionally an alkali reserve. Further details can be found, for example, in the Article by P. A. Boyd et al., “New base oil used in low toxicity oil muds”, Journal of Petroleum Technology, 1985, 137-142.
A key criterion for evaluating the usefulness of invert drilling fluids in practice are their rheological characteristics. Certain viscosity values have to be maintained in drilling fluid systems suitable for practical application. In particular, uncontrolled thickening and hence increases in the viscosity of the drilling fluid have to be strictly prevented because, otherwise, the pipe can become stuck during drilling (cf. Manual of Drilling Fluids Technology, NL Baroid/NL Inc. 1985, Chapter “Stuck Pipe”) and can only be freed by expensive, time-consuming measures. In practice, therefore, suitable diluents are added to the drilling fluid systems before and also during drilling. It is known from the prior art that anionic surfactants from the group of fatty alcohol sulfates, fatty alcohol ether sulfates and alkyl benzenesulfonates are preferably used for this purpose. It has been found in practice, however, that although compounds of this type can effectively influence the rheology of the drilling fluids, problems can arise with diluents known from the prior art when drilling fluids are used at low external temperatures. This applies in particular at temperatures of 50° F. (10° C.) and lower. There is invariably an increase in viscosity which is difficult or impossible to control, even when known diluents are used.
In addition, it is important to ensure that the drilling fluid which is pumped into the ground is heated, for example, to temperatures of 150 to 250° F. (66 or 121° C.), depending on the depth, and—in the case of very deep wells—to temperatures of up to 350° F. (178° C.), although it is not always desirable for the rheology of the drilling fluid at high temperatures to be influenced at the same time. Instead, only selective influencing in the critical low temperature range is desirable in many cases.
In addition, all additives and auxiliaries used in offshore and onshore drilling fluid systems are expected to satisfy stringent biodegradability and toxicity requirements. Also, the ambient conditions prevailing during drilling operations, such as high temperatures, high pressures, changes in pH caused by the inrush of acidic gases, etc., impose high demands on the choice of possible components and additives.
If water-based drilling fluid systems in emulsion form are used, as is often the case today, the presence of emulsifiers is essential. A large number of suitable compounds are known to the expert, for example from the disclosure of EP 0 948 577, of which the technical teaching is confined to special temperature-dependent emulsions. However, a large number of emulsifiers suitable for use in drilling fluids are mentioned in that document, cf. the disclosure of paragraphs 0066 to 0076 of EP 0 948 577 B1.
The choice of emulsifiers for well servicing systems and, more particularly, drilling fluids is primarily directed at finding substances which lead to maximum stability of the emulsion, even under the extreme conditions of practical application, i.e. an increase in the viscosity of the drilling fluid and, more particularly, breaking of the emulsion should be strictly prevented. This applies in particular to emulsions of the water-in-oil type.